The present invention relates to a process for producing silicon carbide whiskers and more particularly to a process for producing silicon carbide whiskers of higher quality and high yield. Silicon carbide in the form of whiskers with a high aspect ratio of length to diameter has been proposed for use as a reinforcement in metals and a toughener in ceramics and thereby greatly enhance their utility.
Considerable scientific study of silicon carbide whiskers has been done and their properties have been found to be of great utility in reinforcing and toughening solids but commercial development has been limited by the extremely high cost of production.
Silicon carbide whiskers have been reported produced by a variety of techniques, however the most significant are the Tateho process, the Tokai process and the ARCO process.
The Tokai process as taught by Yamamoto, U.S. Pat. No. 4,500,504 involves exchanging a water soluble catalyst on to a silica gel and mixing the dried silica gel with a very fine particle carbon black in a 5.5 to 100 C/SiO.sub.2 molar ratio and reacting the mixture in an non-oxidative atmosphere at a temperature between 1300.degree. C. and 1700.degree. C. in an enclosed container in an electric furnace to produce silicon carbide whiskers. In addition 200% by weight sodium chloride is added to the mixture to produce longer and more uniform whiskers. The reaction time is on the order of two to four hours.
The Tateho process, taught by Tanaka et. al. U.S. Pat. No. 4,504,453, involves thin layers of porous silicon and carbon containing feedstock on a series of trays moved through a series of heating zones which successively heat the feedstock, to 1300.degree. C. while a non-oxidizing gas is purged through the feedstock in each tray to remove impurities. Once purified the feedstock is again moved in stages through a series of temperature zones from about 1350.degree. C. to 1450.degree. C. to effect whisker growth. The preferred time in the high temperature zone (1350.degree. to 1450.degree.) is four to ten hours.
Although much is not known about the ARCO process it is believed to be a process involving the heating of coked rice hulls at a temperature above 1600.degree. C.
Cutler U.S. Pat. No. 4,118,464 describes a process for preparing a two phase mixture of silica and carbon in a stochiometric molar ratio, reacting the silica/carbon mixture to produce SiO vapor and transporting the SiO vapor from the first phase into a second phase consisting primarily of pure carbon. To produce silicon carbide whiskers in the second location this reactionis carried out above 1200.degree. C. and iron is thought to accelerate the reaction.
Also of interest is Hayashi et al U.K. patent application GB No. 2,162,504A, assigned to Nippon Light Metal Company.
This process involves a mixture of carbon and silica with a controlled bulk density between 0.03 and 0.15 g/cm.sup.3 in the absence of catalyst. This mixture is placed in containers which are either sealed or vented to the outside and heated to 1500.degree. to 2000.degree. C. under an inert atmosphere as it is lowered through a push furnace.
One of the problems unsolved until the present invention was obtaining a reasonable yield of high quality SiC whiskers, in reasonable reaction times to be economically practical, having an average diameter by mass basis greater than or equal to 0.75 micron at high yields without simultaneously producing hard to remove silicon carbide particulae.